Many electronic devices and optoelectronic devices are based on the GaN material family. These devices require at least one interface between an n-type doped material and a p-type doped material, in order to form a p:n junction and/or allow injection of electrical carriers into the device. GaN is naturally an n-type doped semiconductor material, and p-type doped GaN is obtained by introducing a suitable dopant species during the GaN growth process. Magnesium is often used as a p-type dopant for GaN. Many devices require a free carrier concentration in the p-type doped GaN of at least 1018 cm−3, however, and there have been difficulties in obtaining such carrier concentrations in magnesium-doped GaN. This is because only a few percent of magnesium dopant atoms are electrically activated, and the un-activated dopant atoms do not give rise to a free charge carrier. Thus, incorporating magnesium atoms into a semiconductor material may not achieve p-type doping of the material, and a magnesium-doped semiconductor material may even be n-type.
The epitaxial growth of Group III nitride semiconductor materials on a substrate can be effected by molecular beam epitaxy (MBE) or by chemical vapour deposition (CVD) which is sometimes known as Vapour Phase Epitaxy (VPE).
CVD (or VPE) takes place in an apparatus which is commonly at atmospheric pressure but sometimes at a slightly reduced pressure of typically about 10 kPa. Ammonia and the species providing one or more Group III elements to be used in epitaxial growth are supplied, using a carrier gas, substantially parallel to the surface of a substrate upon which epitaxial growth is to take place, thus forming a boundary layer adjacent to and flowing across the substrate surface. It is in this gaseous boundary layer that decomposition to form nitrogen and the other elements to be epitaxially deposited takes place so that the epitaxial growth is driven by gas phase equilibria.
In contrast to CVD, MBE is carried out in a high vacuum environment. In the case of MBE as applied to the GaN system, an ultra-high vacuum (UHV) environment, typically around 1×10−3 Pa, is used. Ammonia or another nitrogen precursor is supplied to the MBE chamber by means of a supply conduit and a species providing gallium and, possibly, indium and/or aluminium and/or a dopant species are supplied from appropriate sources within heated effusion cells fitted with controllable shutters to control the amounts of the species supplied into the MBE chamber during the epitaxial growth period. The shutter-control outlets from the effusion cells and the nitrogen supply conduit face the surface of the substrate upon which epitaxial growth is to take place. The ammonia and the species supplied from the effusion cells travel across the MBE chamber and reach the substrate where epitaxial growth takes place in a manner which is driven by the deposition kinetics.
At present, the majority of growth of high quality GaN layers is carried out using the metal-organic chemical vapour deposition (MOCVD) process. The MOCVD process allows good control of the growth of the nucleation layer and of the annealing of the nucleation layer. Furthermore, the MOCVD process allows growth to occur at a V/III ratio well in excess of 1000:1. The V/III ratio is the molar ratio of the group V element to the Group III element during the growth process. A high V/III ratio is preferable, since this allows a higher substrate temperature to be used which in turn leads to a higher quality GaN layer.
At present, growing high quality GaN layers by MBE is more difficult than growing such layers by MOCVD. The principal difficulty is in supplying sufficient nitrogen during the growth process. The two commonly used sources of nitrogen in the MBE growth of nitride layers are plasma excited molecular nitrogen or ammonia.
There have been several reports of the growth of p-type doped GaN by MOCVD, such as, for example U.S. Pat. No. 5,306,662 and H Amano et al in “Japanese Journal of Applied Physics” Part 2, Vol. 28 L2112 (1989). It has generally been found that the magnesium dopant atoms in magnesium-doped GaN grown by MOCVD are inactive, so that post-growth processing is required to activate the magnesium atoms in order to generate free charge carriers. This is because magnesium atoms are passivated if the growth process is carried out in the presence of hydrogen. Large quantities of hydrogen are present in the growth of GaN by MOCVD (arising from the hydrogen carrier gas, and from the decomposition of ammonia gas if this is used as the nitrogen source), and these tend to passivate magnesium-doped GaN. It has generally been found necessary to activate the magnesium-doped GaN grown by MOCVD to obtain a reasonable density of free charge carriers, for example by annealing the material or by irradiating the material with a low energy electron beam.
U.S. Pat. No. 6,043,140 discloses a method of growing magnesium-doped GaN by MOCVD using amine gases as the nitrogen source. Magnesium-doped GaN grown using this method is found to have an acceptable electrical conductivity without the need for an annealing step, but this particular growth method is not relevant to MBE growth.
There have been a number of reports of MBE growth of magnesium-doped GaN that do not require a post-growth annealing or irradiation step. The MBE process does not use hydrogen carrier gas, so that the hydrogen level in a MBE growth system is generally lower than the hydrogen level in a MOCVD growth system; as a result passivation of the obtained magnesium-doped GaN is less of a problem in MBE growth than in MOCVD growth. In particular, many reports of MBE growth of GaN use an activated nitrogen plasma source as the nitrogen precursor rather than ammonia, and this eliminates the presence of hydrogen arising from the decomposition of ammonia.
There have been reports by W Kim et al, in “Applied Physics Letters” Vol 69, p 559 (1996), and by M Leroux et al, in “Journal of Applied Physics” Vol 86, p 3721 (1999), of MBE growth of magnesium-doped GaN in which ammonia is used as the nitrogen precursor. In this case, there will be some hydrogen present during the growth process, since hydrogen gas will be formed from the decomposition of the ammonia. These reports achieve free charge concentrations in the range 3-4.5×1017 cm−3. However, in both cases the growth temperatures were no higher than 830° C.
G. Namkoong et al. disclose, in “Applied Physics Letters”, Vol. 77, No. 26, pp4386-4388 (2000), the incorporation of magnesium into gallium nitride grown by plasma-assisted MBE. However, the growth conditions used are such that, although magnesium is incorporated into the gallium nitride, p-type doping is not shown to be achieved.